docker (compose): test environment variable + network alias:

`SAT_TEST_ENV_E2E` is set to `1` to indicate that e2e environment requirement are
available (notably specific accounts and hosts created). The e2e tests needing it won't be
launched if this variable is not set.

A network alias is set for the backend at `sat.test` (to have a tld available, needed for
some network libraries like OpenSSL).

docker/docker-compose_e2e.yml

@@ -40,3 +40,9 @@ services:
   sat:
     build: backend_e2e
     image: salutatoi/sat_e2e
+    environment:
+      SAT_TEST_ENV_E2E: "1"
+    networks:
+      default:
+        aliases:
+          - sat.test